Integrated waste heat recovery in liquefied natural gas facility

ABSTRACT

The present invention relates to a process and apparatus for liquefying natural gas. In another aspect the present invention relates to the heat recovery from a turbine in a liquefied natural gas facility.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit under 35 U.S.C. Section 119(e)to U.S. Provisional Patent Ser. No. 61/443,523 filed on Feb. 16, 2011the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a process and apparatus for liquefyingnatural gas. In another aspect the present invention relates to the heatrecovery from a turbine in a liquefied natural gas facility.

BACKGROUND OF THE INVENTION

The cryogenic liquefaction of natural gas is routinely practiced as ameans of converting natural gas into a more convenient form fortransportation and storage. Such liquefaction reduces the volume of thenatural gas by about 600-fold and results in a product which can bestored and transported at or near atmospheric pressure.

Natural gas is frequently transported by pipeline from the supply sourceto a distant market. It is desirable to operate the pipeline under asubstantially constant and high load factor. However, often thedeliverability or capacity of the pipeline will exceed demand while atother times the demand may exceed the deliverability of the pipeline. Inorder to shave off the peaks where demand exceeds supply or the valleyswhen supply exceeds demand, it is desirable to store the excess gas insuch a manner that it can be delivered when demand exceeds supply. Suchpractice allows future demand peaks to be met with material fromstorage. One practical means for doing this is to convert the gas to aliquefied state for storage and to then vaporize the liquid as demandrequires.

The liquefaction of natural gas is of even greater importance whentransporting gas from a supply source, which is separated by greatdistances from the candidate market and a pipeline either is notavailable or is impractical. This is particularly true where transportmust be made by ocean-going vessel. Ship transportation in the gaseousstate is generally not practical because appreciable pressurization isrequired to significantly reduce the specific volume of the gas. Suchpressurization requires the use of more expensive storage containers.

In order to store and transport natural gas in the liquid state, thenatural gas is preferably cooled to −240° F. to −260° F. where theliquefied natural gas (LNG) possesses a near-atmospheric vapor pressure.Numerous systems exist in the prior art for the liquefaction of naturalgas in which the gas is liquefied by sequentially passing the gas at anelevated pressure through a plurality of cooling stages whereupon thegas is cooled to successively lower temperatures until the liquefactiontemperature is reached. Cooling is generally accomplished by indirectheat exchange with one or more refrigerants such as propane, propylene,ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations ofthe preceding refrigerants (e.g., mixed refrigerant systems).

In one example of the prior art, cooling is accomplished with a seriesof refrigerants. Such refrigerants, for example, can be categorized as aheavy refrigerant, an intermediate refrigerant and a light refrigerant.Each refrigerant is individually utilized in separate refrigerationcycles beginning with the heavy refrigerant and ending with the lightrefrigerant to condense the natural gas and produce LNG. Furthermore,each cycle has its own compressor drive (traditionally using gasturbines but could equally use electric drives powered by gas turbinegenerators). The “first” refrigeration cycle utilizes the heavyrefrigerant to cool the natural gas, the “second” refrigeration cycleutilizes the intermediate refrigerant to further cool the natural gas,and the “final” refrigeration cycle utilizes the light refrigerant tofurther cool the natural gas.

In another prior art example, the integration of economizing exchangersare utilized to facilitate the removal of heat from the natural gasusing sensible heat in the intermediate and light refrigeration systemsrather than the latent heat of condensation in the heavy refrigerationsystems.

Typically, the process for the production of LNG, as described above,requires substantial energy consumption for cooling and liquefaction ofthe natural gas. This energy is supplied by mechanical drives that useprime movers, such as gas turbines, gas engines and/or electric motors,to drive compressors for the necessary refrigeration processes. Theprime movers are inherently inefficient and are known to typicallyconvert only 25-40% of the energy supplied as fuel into usefulcompressive work for the refrigeration process. The majority of energyis lost to atmosphere in the form of heat.

Therefore, a need exits to utilize the energy lost during therefrigeration process and improve the overall thermal efficiency of theprocess.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, a process for liquefyingnatural gas includes the following steps: (a) compressing a firstrefrigerant in a first compressor driven by a first gas turbine; (b)recovering waste heat from the first gas turbine; (c) cooling thenatural gas with the first refrigerant in a first chiller; (d) using atleast a portion of the waste heat recovered from the first gas turbineto further cool the natural gas with the first refrigerant, wherein thenatural gas and the first refrigerant are further cooled by utilizingeither a lithium bromide or an ammonia absorption chiller; (e)compressing a second refrigerant in a second compressor driven by asecond gas turbine; (f) recovering waste hear from the second turbine;(g) cooling the natural gas with the second refrigerant in a secondchiller; and (h) using at least a portion of the waste heat recoveredfrom the second gas turbine to further cool the natural gas with thesecond refrigerant, wherein the natural gas and the second refrigerantare further cooled by utilizing either a lithium bromide or an ammoniaadsorption chiller.

In another embodiment of the present invention, a process for liquefyingnatural gas includes the following steps: (a) compressing a firstrefrigerant in a first compressor driven by a first gas turbine; (b)recovering waste heat from the first gas turbine; (c) using at least aportion of the waste heat recovered from the first gas turbine tofurther cool the natural gas with the first refrigerant; (d) compressinga second refrigerant in a second compressor driven by a second gasturbine; (e) recovering waste heat from the second turbine; and (f)using at least a portion of the waste heat recovered from the second gasturbine to further cool the natural gas with the second refrigerant.

In another embodiment, a process for liquefying natural gas, the processincludes the following steps: (a) compressing a first refrigerant in afirst compressor driven by a first gas turbine; (b) recovering wasteheat from the first gas turbine; (c) using at least a portion of thewaste heat recovered from the first gas turbine to further cool thenatural gas; (d) compressing a second refrigerant in a second compressordriven by a second gas turbine; (e) recovering waste heat from thesecond turbine; and (f) using at least a portion of the waste heatrecovered from the second gas turbine to further cool the natural gas.

In yet another embodiment of the present invention, an apparatus forliquefying natural gas employing multiple refrigerants in multiplerefrigeration cycle for cooling the natural gas in multiple stage,includes: (a) a first compressor for compressing a first refrigerant ofa first refrigeration cycle; (b) a first gas turbine for driving thefirst compressor, wherein waste heat is recovered from the first gasturbine, wherein at least a portion of the waste heat further cools thenatural gas and the first refrigerant; (c) a second compressor forcompressing a second refrigerant of a second refrigeration cycle; and(d) a second gas turbine for driving the second compressor, whereinwaste heat is recovered from the second gas turbine, wherein at least aportion of the waste heat further cools the natural gas and the secondrefrigerant.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further advantages thereof, may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a simplified flow diagram of a cascaded refrigeration processfor LNG production which employs an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to embodiments of the presentinvention, one or more examples of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation ofthe invention, not as a limitation of the invention. It will be apparentto those skilled in the art that various modifications and variationscan be made in the present invention without departing from the scope orspirit of the invention. For instance, features illustrated or describedas part of one embodiment can be used on another embodiment to yield astill further embodiment. Thus, it is intended that the presentinvention cover such modifications and variations that come within thescope of the appended claims and their equivalents.

A cascade refrigeration process uses one or more refrigerants fortransferring heat energy from the natural gas stream to the refrigerantand ultimately transferring said heat energy to the environment. Inessence, the overall refrigeration system functions as a heat pump byremoving heat energy from the natural gas stream as the stream isprogressively cooled to lower and lower temperatures. The design of acascade refrigeration process involves the balancing of thermodynamicefficiencies and capital costs. In heat transfer processes,thermodynamic irreversibilities are reduced as the temperature gradientsbetween heating and cooling fluids become smaller. However, obtainingsuch small temperature gradients generally requires (1) significantincreases in the amount of heat transfer area; (2) major modificationsto various process equipment; and (3) the proper selection of flowratesthrough such equipment so as to ensure both flowrates, approachtemperatures and outlet temperatures are compatible with the requiredheating/cooling duty.

One of the most efficient and effective means of liquefying natural gasis via an optimized cascade-type operation in combination withexpansion-type cooling. Such a liquefaction process involves thecascade-type cooling of a natural gas stream at an elevated pressure,(e.g., about 650 psia) by sequentially cooling the gas stream viapassage through a multistage propane or propylene cycle, a multistageethane or ethylene cycle, and an open-loop methane cycle which utilizesa portion of the feed gas as a source of methane and which includestherein a multistage expansion cycle to further cool the same and reducethe pressure to near-atmospheric pressure. In another embodiment, themethane cycle can be a closed loop system. In the sequence of coolingcycles, the refrigerant having the highest boiling point is utilizedfirst followed by a refrigerant having an intermediate boiling point andfinally by a refrigerant having the lowest boiling point. As usedherein, the terms “upstream” and “downstream” shall be used to describethe relative positions of various components of a natural gasliquefaction plant along the flow path of natural gas through the plant.

In cryogenic processing of a natural gas stream an importantconsideration is contamination. The raw natural gas feed stream suitablefor the process of the invention may comprise natural gas obtained froma crude oil well (associated gas) or from a gas well (non-associatedgas). The composition of natural gas can vary significantly. Whilemethane is the major desired component of natural gas streams, thetypical raw natural gas stream also contains ethane (C₂), higherhydrocarbons (C₃+), and minor amounts of contaminants such as water,carbon dioxide, hydrogen sulfide, nitrogen, butane, hydrocarbons of sixor more carbon atoms, dirt, iron sulfide, wax, and crude oil. Thesolubilities of these contaminants vary with temperature, pressure, andcomposition. At cryogenic temperatures, CO₂, water, and othercontaminants can form solids, which can plug flow passages in cryogenicheat exchangers.

Various pretreatment steps provide a means for removing undesirablecomponents, such as acid-gases, mercaptan, mercury, and moisture fromthe natural gas feed stream delivered to the LNG facility. Thecomposition of this natural gas stream may vary significantly. As usedherein, a natural gas stream is any stream principally comprised ofmethane which originates in major portion from a natural gas feed streamwith the balance being ethane, higher hydrocarbons, nitrogen, carbondioxide and minor amounts of other contaminants such as mercury,hydrogen sulfide, and mercaptan. The pretreatment steps may be separatesteps located either upstream of the cooling cycles or locateddownstream of one of the early stages of cooling in the initial cycle.The following is a non-exclusive listing of some of the available meanswhich are readily available to one skilled in the art: (a) acid gasesand to a lesser extent mercaptan are routinely removed via a sorptionprocess employing an aqueous amine-bearing solution; (b) a major portionof the water is routinely removed as a liquid via two-phase gas-liquidseparation following gas compression and cooling upstream of the initialcooling cycle and also downstream of the first cooling stage in theinitial cooling cycle; (c) mercury is routinely removed via mercurysorbent beds; and (d) residual amounts of water and acid gases areroutinely removed via the use of properly selected sorbent beds such asregenerable molecular sieves.

As previously noted, the natural gas feed stream is cooled in aplurality of multistage (for example, three) cycles or steps by indirectheat exchange with a plurality of refrigerants, preferably three. Theoverall cooling efficiency for a given cycle improves as the number ofstages increases but this increase in efficiency is accompanied bycorresponding increases in net capital cost and process complexity. Thenatural gas feed stream is preferably passed through an effective numberof refrigeration stages, nominally two, preferably two to four, and morepreferably three stages, in the first refrigeration cycle, also referredherein as the first cooling cycle, utilizing a first refrigerant havingrelatively high boiling refrigerant. Such refrigerant is preferablycomprised in major portion of propane, propylene or mixtures thereof,more preferably the refrigerant comprises at least about 75 mole percentpropane, even more preferably at least 90 mole percent propane, and mostpreferably the refrigerant consists essentially of propane.

Thereafter, the processed natural gas feed stream flows through aneffective number of stages, nominally two, preferably two to four, andmore preferably two or three, in a second refrigeration cycle, alsoreferred herein as the second cooling cycle, in heat exchange with asecond refrigerant having a lower boiling point. Such refrigerant ispreferably comprised in major portion of ethane, ethylene or mixturesthereof, more preferably the refrigerant comprises at least about 75mole percent ethylene, even more preferably at least 90 mole percentethylene, and most preferably the refrigerant consists essentially ofethylene. As previously noted, the processed natural gas feed stream iscombined with one or more recycle streams at various locations in thesecond cycle thereby producing a liquefaction stream. In the last stageof the second cooling cycle, the liquefaction stream is condensed (i.e.,liquefied) in major portion, preferably in its entirety therebyproducing a pressurized LNG-bearing stream. Generally, the processpressure at this location is only slightly lower than the pressure ofthe pretreated natural gas feed stream of the first stage of the firstcycle.

Thereafter, the processed natural gas feed stream flows through aneffective number of stages, nominally two, preferably two to four, andmore preferably three, in a final refrigeration cycle in indirect heatexchange with a final refrigerant. The final refrigerant consistsessentially of methane. In a particularly preferred embodiment, thepredominately methane refrigerant comprises less than 10 mole percentnitrogen, most preferably less than 5 mole percent nitrogen. Eachcooling stage comprises a separate cooling zone.

Generally, the natural gas feed stream will contain such quantities ofC₂+ components so as to result in the formation of a C₂+ rich liquid inone or more of the cooling cycles. This liquid is removed via gas-liquidseparation means, preferably one or more conventional gas-liquidseparators. Generally, the sequential cooling of the natural gas streamin each stage is controlled so as to remove as much as possible of theC₂ and higher molecular weight hydrocarbons from the gas to produce agas stream predominating in methane and a liquid stream containingsignificant amounts of ethane and heavier components. An effectivenumber of gas/liquid separation means are located at strategic locationsdownstream of the cooling zones for the removal of liquids streams richin C₂+ components. The exact locations and number of gas/liquidseparation means, preferably conventional gas/liquid separators, will bedependant on a number of operating parameters, such as the C₂+composition of the natural gas feed stream, the desired BTU content ofthe LNG product, the value of the C₂+ components for other applicationsand other factors routinely considered by those skilled in the art ofLNG plant and gas plant operations. The C₂+ hydrocarbon stream orstreams may be demethanized via a single stage flash or a fractionationcolumn. In the latter case, the resulting natural gas stream can bedirectly returned at pressure to the liquefaction process. In the formercase, this natural gas stream can be repressurized and recycled or canbe used as fuel gas. The C₂+ hydrocarbon stream or streams or thedemethanized C₂+ hydrocarbon stream may be used as fuel or may befurther processed such as by fractionation in one or more fractionationzones to produce individual streams rich in specific chemicalconstituents (e.g., C₂, C₃, C₄ and C₅+).

The liquefaction process may use one of several types of cooling whichinclude but is not limited to (a) indirect heat exchange, (b)vaporization, and (c) expansion or pressure reduction. Indirect heatexchange, as used herein, refers to a process wherein the refrigerantcools the substance to be cooled without actual physical contact betweenthe refrigerating agent and the substance to be cooled. Specificexamples of indirect heat exchange means include heat exchange in ashell-and-tube heat exchanger, a core-in-kettle heat exchanger, and abrazed aluminum plate-fin heat exchanger. The physical state of therefrigerant and substance to be cooled can vary depending on the demandsof the system and the type of heat exchanger chosen. Thus, ashell-and-tube heat exchanger will typically be utilized where therefrigerating agent is in a liquid state and the substance to be cooledis in a liquid or gaseous state or when one of the substances undergoesa phase change and process conditions do not favor the use of acore-in-kettle heat exchanger. As an example, aluminum and aluminumalloys are preferred materials of construction for the core but suchmaterials may not be suitable for use at the designated processconditions. A plate-fin heat exchanger will typically be utilized wherethe refrigerant is in a gaseous state and the substance to be cooled isin a liquid or gaseous state. Finally, the core-in-kettle heat exchangerwill typically be utilized where the substance to be cooled is liquid orgas and the cool stream undergoes a phase change from a liquid state toa gaseous state during the heat exchange.

Vaporization cooling refers to the cooling of a substance by theevaporation or vaporization of a portion of the substance with thesystem maintained at a constant pressure. Thus, during the vaporization,the portion of the substance which evaporates absorbs heat from theportion of the substance which remains in a liquid state and hence,cools the liquid portion.

Finally, expansion or pressure reduction cooling refers to cooling whichoccurs when the pressure of a gas, liquid or a two-phase system isdecreased by passing through a pressure reduction means. In oneembodiment, this expansion means is a Joule-Thomson expansion valve. Inanother embodiment, the expansion means is either a hydraulic or gasexpander. Because expanders recover work energy from the expansionprocess, lower process stream temperatures are possible upon expansion.

Referring to FIG. 1, during normal operation of the LNG facility,gaseous propane is compressed in a multistage (preferably three-stage)compressor 18 driven by a gas turbine driver (not illustrated). Thethree stages of compression preferably exist in a single unit althougheach stage of compression may be a separate unit. The compressed propaneis discharged from propane compressor 18 and then passed through conduit300 to propane condenser 20, wherein the stream is cooled and liquefiedvia indirect heat exchanger with an external fluid (e.g., air or water).A representative pressure and temperature of the liquefied propanerefrigerant prior to flashing is about 100° F. and about 190 psia. Thestream in conduit 302 exits propane condenser 20, whereupon the streamis passed to a propane waste heat chiller 15, wherein the stream issubcooled via indirect heat exchanger with an external fluid (e.g.,lithium bromide or ammonia). The additional chilling step driven in partby waste heat from the liquefaction step allows condensing of thepropane refrigerant at a lower temperature and pressure. The impact ofcondensing at a lower pressure is a reduction in propane compressionhorsepower required for a given amount of refrigeration and to improvethe overall thermal efficiency of the process. Upon exiting propanewaste heat chiller 15 via conduit 301, the stream enters propaneaccumulator 13. A predominately liquid propane refrigerant stream exitspropane accumulator 13 via conduit 305 and is delivered to a pressurereduction means, illustrated as expansion valve 12, wherein the pressureof the liquefied propane is reduced, thereby evaporating or flashing aportion thereof. The resulting two-phase product then flows throughconduit 304 into a high-stage propane chiller 2 wherein gaseous methanerefrigerant introduced via conduit 152, natural gas feed introduced viaconduit 100, and gaseous ethylene refrigerant introduced via conduit 202are respectively cooled via indirect heat exchange means 4, 6, and 8,thereby producing cooled gas streams respectively produced via conduits154, 102, and 204. The gas in conduit 154 is fed to a main methaneeconomizer 74, which will be discussed in greater detail in a subsequentsection, and wherein the stream is cooled via indirect heat exchangemeans 97. A portion of the stream cooled in heat exchange means 97 isremoved from methane economizer 74 via conduit 155 and subsequentlyused, after further cooling, as a reflux stream in a heavies removalcolumn 60. The portion of the cooled stream from heat exchange means 97that is not removed for use as a reflux stream is further cooled inindirect heat exchange means 98. The resulting cooled methane recyclestream produced via conduit 158 is then combined in conduit 120 with theheavies depleted (i.e., light-hydrocarbon rich) vapor stream fromheavies removal column 60 and fed to an ethylene condenser 68.

The propane gas from chiller 2 is returned to compressor 18 throughconduit 306. This gas is fed to the high-stage inlet port of compressor18. The remaining liquid propane is passed through conduit 308, thepressure further reduced by passage through a pressure reduction means,illustrated as expansion valve 14, whereupon an additional portion ofthe liquefied propane is flashed. The resulting two-phase stream is thenfed to an intermediate stage propane chiller 22 through conduit 310,thereby providing a coolant for chiller 22. The cooled feed gas streamfrom chiller 2 flows via conduit 102 to a knock-out vessel 10 whereingas and liquid phases are separated. The liquid phase, which is rich inC₃+ components, is removed via conduit 103. The gaseous phase is removedvia conduit 104 and then split into two separate streams which areconveyed via conduits 106 and 108. The stream in conduit 106 is fed topropane chiller 22. The stream in conduit 108 is employed as a strippinggas in refluxed heavies removal column 60 to aid in the removal of heavyhydrocarbon components from the processed natural gas stream. Ethylenerefrigerant from chiller 2 is introduced to chiller 22 via conduit 204.In chiller 22, the feed gas stream, also referred to herein as amethane-rich stream, and the ethylene refrigerant streams arerespectively cooled via indirect heat transfer means 24 and 26, therebyproducing cooled methane-rich and ethylene refrigerant streams viaconduits 110 and 206. The evaporated portion of the propane refrigerantis separated and passed through conduit 311 to the intermediate-stageinlet of compressor 18. Liquid propane refrigerant from chiller 22 isremoved via conduit 314, flashed across a pressure reduction means,illustrated as expansion valve 16, and then fed to a low-stage propanechiller/condenser 28 via conduit 316.

As illustrated in FIG. 1, the methane-rich stream flows fromintermediate-stage propane chiller 22 to the low-stage propanechiller/condenser 28 via conduit 110. In chiller 28, the stream iscooled via indirect heat exchange means 30. In a like manner, theethylene refrigerant stream flows from the intermediate-stage propanechiller 22 to low-stage propane chiller/condenser 28 via conduit 206. Inthe latter, the ethylene refrigerant is totally condensed or condensedin nearly its entirety via indirect heat exchange means 32. Thevaporized propane is removed from low-stage propane chiller/condenser 28and returned to the low-stage inlet of compressor 18 via conduit 320.

As illustrated in FIG. 1, the methane-rich stream exiting low-stagepropane chiller 28 is introduced to high-stage ethylene chiller 42 viaconduit 112. Ethylene refrigerant exits low-stage propane chiller 28 viaconduit 208 and is preferably fed to a separation vessel 37 whereinlight components are removed via conduit 209 and condensed ethylene isremoved via conduit 210. The ethylene refrigerant at this location inthe process is generally at a temperature of about −24° F. and apressure of about 285 psia. The ethylene refrigerant then flows to anethylene economizer 34 wherein it is cooled via indirect heat exchangemeans 38, removed via conduit 211, and passed to a pressure reductionmeans, illustrated as an expansion valve 40, whereupon the refrigerantis flashed to a preselected temperature and pressure and fed tohigh-stage ethylene chiller 42 via conduit 212. Vapor is removed fromchiller 42 via conduit 214 and routed to ethylene economizer 34 whereinthe vapor functions as a coolant via indirect heat exchange means 46.The ethylene vapor is then removed from ethylene economizer 34 viaconduit 216 and feed to the high-stage inlet of ethylene compressor 48.The ethylene refrigerant which is not vaporized in high-stage ethylenechiller 42 is removed via conduit 218 and returned to ethyleneeconomizer 34 for further cooling via indirect heat exchange means 50,removed from ethylene economizer via conduit 220, and flashed in apressure reduction means, illustrated as expansion valve 52, whereuponthe resulting two-phase product is introduced into a low-stage ethylenechiller 54 via conduit 222.

After cooling in indirect heat exchange means 44, the methane-richstream is removed from high-stage ethylene chiller 42 via conduit 116.The stream in conduit 116 is then carried to a feed inlet of heaviesremoval column 60 wherein heavy hydrocarbon components are removed fromthe methane-rich stream. A heavies-rich liquid stream containing asignificant concentration of C₄+ hydrocarbons, such as benzene, toluene,xylene, cyclohexane, other aromatics, and/or heavier hydrocarboncomponents, is removed from the bottom of heavies removal column 60 viaconduit 114. The heavies-rich stream in conduit 114 is subsequentlyseparated into liquid and vapor portions or preferably is flashed orfractionated in vessel 67. In either case, a second heavies-rich liquidstream is produced via conduit 123 and a second methane-rich vaporstream is produced via conduit 121. In the preferred embodiment, whichis illustrated in FIG. 1, the stream in conduit 121 is subsequentlycombined with a second stream delivered via conduit 128, and thecombined stream fed to the high-stage inlet port of the methanecompressor 83. High-stage ethylene chiller 42 also includes an indirectheat exchanger means 43 which receives and cools the stream withdrawnfrom methane economizer 74 via conduit 155, as discussed above. Theresulting cooled stream from indirect heat exchanger means 43 isconducted via conduit 157 to low-stage ethylene chiller 54. In low-stageethylene chiller 54 the stream from conduit 157 is cooled via indirectheat exchange means 56. After cooling in indirect heat exchange means56, the stream exits low-stage ethylene chiller 54 and is carried viaconduit 159 to a reflux inlet of heavies removal column 60 where it isemployed as a reflux stream.

As previously noted, the gas in conduit 154 is fed to main methaneeconomizer 74 wherein the stream is cooled via indirect heat exchangemeans 97. A portion of the cooled stream from heat exchange means 97 isthen further cooled in indirect heat exchange means 98. The resultingcooled stream is removed from methane economizer 74 via conduit 158 andis thereafter combined with the heavies-depleted vapor stream exitingthe top of heavies removal column 60 and fed to a low-stage ethylenecondenser 68. In low-stage ethylene condenser 68, this stream is cooledand condensed via indirect heat exchange means 70 with the liquideffluent from low-stage ethylene chiller 54 which is routed to low-stageethylene condenser 68 via conduit 226. The condensed methane-richproduct from low-stage condenser 68 is produced via conduit 122. Thevapor from low-stage ethylene chiller 54, withdrawn via conduit 224, andlow-stage ethylene condenser 68, withdrawn via conduit 228, are combinedand routed, via conduit 230, to ethylene economizer 34 wherein thevapors function as a coolant via indirect heat exchange means 58. Thestream is then routed via conduit 232 from ethylene economizer 34 to thelow-stage inlet of ethylene compressor 48.

As noted in FIG. 1, the compressor effluent from vapor introduced viathe low-stage side of ethylene compressor 48 is removed via conduit 234,cooled via inter-stage cooler 71, and returned to compressor 48 viaconduit 236 for injection with the high-stage stream present in conduit216. Preferably, the two-stages are a single module although they mayeach be a separate module and the modules mechanically coupled to acommon driver. The compressed ethylene product from compressor 48 isrouted to a downstream cooler 72 via conduit 200. The product fromcooler 72 flows via conduit 201 and is introduced to an ethylene wasteheat chiller 73 to provide additional cooling to the refrigerant viaindirect heat exchange with an external fluid (e.g., lithium bromide orammonia). Additional sensible heat is removed via the waste heatrefrigeration system due to the lower temperatures available, which hasthe effect of reducing the duty required to be removed in the mainpropane chillers, which can reduce power requirements (or increasecapacity) in the propane system. Upon exiting the ethylene waste heatchiller 73 via conduit 202, the stream is delivered to high-stagepropane chiller 2.

The pressurized LNG-bearing stream, preferably a liquid stream in itsentirety, in conduit 122 is preferably at a temperature in the range offrom about −200 to about −50° F., more preferably in the range of fromabout −175 to about −100° F., most preferably in the range of from −150to −125° F. The pressure of the stream in conduit 122 is preferably inthe range of from about 500 to about 700 psia, most preferably in therange of from 550 to 725 psia. The stream in conduit 122 is directed tomain methane economizer 74 wherein the stream is further cooled byindirect heat exchange means/heat exchanger pass 76 as hereinafterexplained. It is preferred for main methane economizer 74 to include aplurality of heat exchanger passes which provide for the indirectexchange of heat between various predominantly methane streams in theeconomizer 74. Preferably, methane economizer 74 comprises one or moreplate-fin heat exchangers. The cooled stream from heat exchanger pass 76exits methane economizer 74 via conduit 124. It is preferred for thetemperature of the stream in conduit 124 to be at least about 10° F.less than the temperature of the stream in conduit 122, more preferablyat least about 25.degree. F. less than the temperature of the stream inconduit 122. Most preferably, the temperature of the stream in conduit124 is in the range of from about −200 to about −160° F. The pressure ofthe stream in conduit 124 is then reduced by a pressure reduction means,illustrated as expansion valve 78, which evaporates or flashes a portionof the gas stream thereby generating a two-phase stream. The two-phasestream from expansion valve 78 is then passed to high-stage methaneflash drum 80 where it is separated into a flash gas stream dischargedthrough conduit 126 and a liquid phase stream (i.e., pressurizedLNG-bearing stream) discharged through conduit 130. The flash gas streamis then transferred to main methane economizer 74 via conduit 126wherein the stream functions as a coolant in heat exchanger pass 82. Thepredominantly methane stream is warmed in heat exchanger pass 82, atleast in part, by indirect heat exchange with the predominantly methanestream in heat exchanger pass 76. The warmed stream exits heat exchangerpass 82 and methane economizer 74 via conduit 128.

The liquid-phase stream exiting high-stage flash drum 80 via conduit 130is passed through a second methane economizer 87 wherein the liquid isfurther cooled by downstream flash vapors via indirect heat exchangemeans 88. The cooled liquid exits second methane economizer 87 viaconduit 132 and is expanded or flashed via pressure reduction means,illustrated as expansion valve 91, to further reduce the pressure and,at the same time, vaporize a second portion thereof. This two-phasestream is then passed to an intermediate-stage methane flash drum 92where the stream is separated into a gas phase passing through conduit136 and a liquid phase passing through conduit 134. The gas phase flowsthrough conduit 136 to second methane economizer 87 wherein the vaporcools the liquid introduced to economizer 87 via conduit 130 viaindirect heat exchanger means 89. Conduit 138 serves as a flow conduitbetween indirect heat exchange means 89 in second methane economizer 87and heat exchanger pass 95 in main methane economizer 74. The warmedvapor stream from heat exchanger pass 95 exits main methane economizer74 via conduit 140, is combined with the first nitrogen-reduced streamin conduit 406, and the combined stream is conducted to theintermediate-stage inlet of methane compressor 83.

The liquid phase exiting intermediate-stage flash drum 92 via conduit134 is further reduced in pressure by passage through a pressurereduction means, illustrated as an expansion valve 93. Again, a thirdportion of the liquefied gas is evaporated or flashed. The two-phasestream from expansion valve 93 are passed to a final or low-stage flashdrum 94. In flash drum 94, a vapor phase is separated and passed throughconduit 144 to second methane economizer 87 wherein the vapor functionsas a coolant via indirect heat exchange means 90, exits second methaneeconomizer 87 via conduit 146, which is connected to the first methaneeconomizer 74 wherein the vapor functions as a coolant via heatexchanger pass 96. The warmed vapor stream from heat exchanger pass 96exits main methane economizer 74 via conduit 148, is combined with thesecond nitrogen-reduced stream in conduit 408, and the combined streamis conducted to the low-stage inlet of compressor 83.

The liquefied natural gas product from low-stage flash drum 94, which isat approximately atmospheric pressure, is passed through conduit 142 toa LNG storage tank 99. In accordance with conventional practice, theliquefied natural gas in storage tank 99 can be transported to a desiredlocation (typically via an ocean-going LNG tanker). The LNG can then bevaporized at an onshore LNG terminal for transport in the gaseous statevia conventional natural gas pipelines.

As shown in FIG. 1, the high, intermediate, and low stages of compressor83 are preferably combined as single unit. However, each stage may existas a separate unit where the units are mechanically coupled together tobe driven by a single driver. The compressed gas from the low-stagesection passes through an inter-stage cooler 85 and is combined with theintermediate pressure gas in conduit 140 prior to the second-stage ofcompression. The compressed gas from the intermediate stage ofcompressor 83 is passed through an inter-stage cooler 84 and is combinedwith the high pressure gas provided via conduits 121 and 128 prior tothe third-stage of compression. The compressed gas (i.e., compressedopen methane cycle gas stream) is discharged from high stage methanecompressor through conduit 150 and is cooled in cooler 86. The productfrom cooler 86 flows via conduit 151 and is introduced to methane wasteheat chiller 143 to provide additional cooling to the refrigerant viaindirect heat exchange with an external fluid (e.g., lithium bromide orammonia). Additional sensible heat is removed via the waste heatrefrigeration system due to the lower temperatures available. This hasthe effect of reducing the duty required to be removed in the mainpropane chillers, which can reduce power requirements (or increasecapacity) in the propane system. Upon exiting the ethylene waste heatchiller 143 the stream is routed to the high pressure propane chiller 2via conduit 152 as previously discussed. The stream is cooled in chiller2 via indirect heat exchange means 4 and flows to main methaneeconomizer 74 via conduit 154. The compressed open methane cycle gasstream from chiller 2 which enters the main methane economizer 74undergoes cooling in its entirety via flow through indirect heatexchange means 98. This cooled stream is then removed via conduit 158and combined with the processed natural gas feed stream upstream of thefirst stage of ethylene cooling.

The preferred embodiment of the present invention has been disclosed andillustrated. However, the invention is intended to be as broad asdefined in the claims below. Those skilled in the art may be able tostudy the preferred embodiments and identify other ways to practice theinvention that are not exactly as described in the present invention. Itis the intent of the inventors that variations and equivalents of theinvention are within the scope of the claims below and the description,abstract and drawings not to be used to limit the scope of theinvention.

1. A process for liquefying natural gas, the process comprising thesteps of: a. compressing a first refrigerant in a first compressordriven by a first gas turbine; b. recovering waste heat from the firstgas turbine; c. cooling the natural gas with the first refrigerant in afirst chiller; d. using at least a portion of the waste heat recoveredfrom the first gas turbine to further cool the natural gas with thefirst refrigerant, wherein the natural gas and the first refrigerant arefurther cooled by utilizing either a lithium bromide or an ammoniaabsorption chiller; e. compressing a second refrigerant in a secondcompressor driven by a second gas turbine; f. recovering waste hear fromthe second turbine; g. cooling the natural gas with the secondrefrigerant in a second chiller; and h. using at least a portion of thewaste heat recovered from the second gas turbine to further cool thenatural gas with the second refrigerant, wherein the natural gas and thesecond refrigerant are further cooled by utilizing either a lithiumbromide or an ammonia adsorption chiller.
 2. The process according claim1, wherein the first refrigerant comprising in major portion ahydrocarbon selected from a group consisting of propane, propylene,ethane, ethylene, or combinations thereof.
 3. The process according toclaim 1, wherein the first refrigerant comprising in major portionpropane or propylene.
 4. The process according to claim 1, wherein thefirst refrigerant comprises in major portion of ethane or ethylene. 5.The process according to claim 1, wherein the second refrigerantcomprising at least about 75 mole percent methane.
 6. A process forliquefying natural gas, the process comprising the steps of: a.compressing a first refrigerant in a first compressor driven by a firstgas turbine; b. recovering waste heat from the first gas turbine; c.using at least a portion of the waste heat recovered from the first gasturbine to further cool the natural gas with the first refrigerant; d.compressing a second refrigerant in a second compressor driven by asecond gas turbine; e. recovering waste heat from the second turbine;and f. using at least a portion of the waste heat recovered from thesecond gas turbine to further cool the natural gas with the secondrefrigerant.
 7. The process according to claim 6, wherein cooling step(c) utilizes either a lithium bromide or an ammonia absorption chiller.8. The process according to claim 6, wherein cooling step (f) utilizeseither a lithium bromide or an ammonia absorption chiller.
 9. Theprocess according to claim 6, wherein prior to step (c) cooling thenatural gas with the first refrigerant in a first chiller.
 10. Theprocess according to claim 6, wherein prior to step (d) cooling thenatural gas with the second refrigerant in a second chiller.
 11. Theprocess according claim 6, wherein the first refrigerant comprising inmajor portion a hydrocarbon selected from a group consisting of propane,propylene, ethane, ethylene, or combinations thereof.
 12. The processaccording to claim 6, wherein the first refrigerant comprising in majorportion propane or propylene.
 13. The process according to claim 6,wherein the first refrigerant comprising in major portion ethane orethylene.
 14. The process according to claim 6, wherein the secondrefrigerant comprising at least about 75 mole percent methane.
 15. Aprocess for liquefying natural gas, the process comprising the steps of:a. compressing a first refrigerant in a first compressor driven by afirst gas turbine; b. recovering waste heat from the first gas turbine;c. using at least a portion of the waste heat recovered from the firstgas turbine to further cool the natural gas; d. compressing a secondrefrigerant in a second compressor driven by a second gas turbine; e.recovering waste heat from the second turbine; and f. using at least aportion of the waste heat recovered from the second gas turbine tofurther cool the natural gas.
 16. The process according to claim 15,wherein cooling step (c) utilizes either a lithium bromide or an ammoniaabsorption chiller.
 17. The process according to claim 15, whereincooling step (f) utilizes either a lithium bromide or an ammoniaabsorption chiller.
 18. The process according to claim 15, wherein priorto step (c) cooling the natural gas with the first refrigerant in afirst chiller.
 19. The process according to claim 15, wherein prior tostep (d) cooling the natural gas with the second refrigerant in a secondchiller.
 20. The process according claim 15, wherein the firstrefrigerant comprising in major portion a hydrocarbon selected from agroup consisting of propane, propylene, ethane, ethylene, orcombinations thereof.
 21. The process according to claim 15, wherein thefirst refrigerant comprising in major portion propane or propylene. 22.The process according to claim 15, wherein the first refrigerantcomprising in major portion ethane or ethylene.
 23. The processaccording to claim 15, wherein the second refrigerant comprising atleast about 75 mole percent methane.
 24. An apparatus for liquefyingnatural gas, the apparatus employing multiple refrigerants in multiplerefrigeration cycle for cooling the natural gas in multiple stage, theapparatus comprising: a. a first compressor for compressing a firstrefrigerant of a first refrigeration cycle; b. a first gas turbine fordriving the first compressor, wherein waste heat is recovered from thefirst gas turbine, wherein at least a portion of the waste heat furthercools the natural gas and the first refrigerant; c. a second compressorfor compressing a second refrigerant of a second refrigeration cycle;and d. a second gas turbine for driving the second compressor, whereinwaste heat is recovered from the second gas turbine, wherein at least aportion of the waste heat further cools the natural gas and the secondrefrigerant.
 25. The apparatus according claim 24, wherein the firstrefrigerant comprising in major portion a hydrocarbon selected from agroup consisting of propane, propylene, ethane, ethylene, orcombinations thereof.
 26. The apparatus according to claim 24, whereinthe first refrigerant comprising in major portion propane or propylene.27. The apparatus according to claim 24, wherein the first refrigerantcomprising in major portion ethane or ethylene.
 28. The processaccording to claim 24, wherein the second refrigerant comprising atleast about 75 mole percent methane.